The stainless models have steel ejector housings, but most blued ones have almuminum. I think the Super Blackhawks are an exception--I'm sure someone will correct me if I'm wrong, but I believe they're all steel, whether blued or stainless. I think the anniversary model flattop blackhawks are also steel.
I'm sorry I can't help with your second question. The steel housings are out there, but I don't know the best place to get one. I'd probably start by putting a Want to Buy ad in the classifieds here at RugerForum.